CVE-2026-64531 in Linux
Summary
by MITRE • 07/27/2026
In the Linux kernel, the following vulnerability has been resolved:
net: openvswitch: reject oversized nested action attrs
Open vSwitch stores generated flow actions as nlattrs, whose nla_len field is u16. Commit a1e64addf3ff ("net: openvswitch: remove misbehaving actions length check") allowed the total sw_flow_actions stream to grow beyond 64 KiB, which is valid, but also removed the last guard preventing a generated nested action attribute from exceeding U16_MAX.
An oversized generated container can thus be closed with a truncated nla_len. A later dump or teardown then walks a structurally different stream than the one that was validated. In particular, an oversized nested CLONE/CT action may cause subsequent bytes in the generated stream to be interpreted as independent actions.
Keep the larger total-action-stream behavior, but make nested action close reject generated containers that do not fit in nla_len, and return the error through all callers. For recursive SAMPLE, CLONE, DEC_TTL, and CHECK_PKT_LEN builders, trim resource-owning action-list tails in reverse construction order before discarding failed wrappers, so resources copied into the rejected tails are released before the wrappers are removed.
Most failed outer wrappers are discarded by truncating actions_len after child resources have been released. CHECK_PKT_LEN also trims its parent after branch resources are gone. SET/TUNNEL close failures unwind their known tun_dst ownership directly, and SET_TO_MASKED has no external ownership and truncates on close failure.
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Analysis
by VulDB Data Team • 07/28/2026
The vulnerability in the Linux kernel's Open vSwitch implementation stems from improper handling of nested action attributes that can exceed the maximum allowed size for netlink attribute length fields. This issue affects the network virtualization infrastructure used by many cloud and containerized environments, where Open vSwitch serves as a key component for software-defined networking. The problem emerged following a specific commit that removed a critical validation check, allowing flow action streams to grow beyond traditional 64 KiB limits while simultaneously eliminating protection against nested attributes exceeding U16_MAX values.
The technical flaw manifests when processing generated flow actions stored as nlattrs with u16 nla_len fields. When oversized nested action containers are created, they can be closed with truncated nla_len values that don't accurately represent the actual data structure size. This structural inconsistency creates a dangerous condition where subsequent operations like dump or teardown processes encounter a different stream structure than what was originally validated. The vulnerability particularly impacts CLONE and CT actions which can cause subsequent bytes in the generated stream to be misinterpreted as independent actions, potentially leading to memory corruption or arbitrary code execution.
This vulnerability directly maps to CWE-129, which addresses insufficient validation of length fields, and CWE-787, concerning out-of-bounds write conditions. From an operational perspective, this flaw represents a significant risk in environments where Open vSwitch handles untrusted network traffic, as attackers could potentially craft malicious flow actions that exploit this behavior to cause system instability or privilege escalation. The issue affects the integrity of the kernel's networking subsystem and can compromise the overall security posture of systems relying on software-defined networking.
The mitigation strategy involves maintaining the existing capability to handle larger total action streams while implementing stricter validation for nested action attributes. The solution requires rejecting generated containers that exceed nla_len capacity and propagating these errors through all affected callers. For recursive actions including SAMPLE, CLONE, DEC_TTL, and CHECK_PKT_LEN, the implementation must trim resource-owning action-list tails in reverse construction order before discarding failed wrappers to ensure proper resource cleanup. This approach prevents memory leaks and maintains system stability while preserving the legitimate functionality of handling larger action streams.
The fix addresses specific edge cases in different action types through targeted approaches for resource management. For CHECK_PKT_LEN actions, the implementation trims parent structures after branch resources have been released, ensuring complete cleanup before structural modifications occur. SET/TUNNEL actions handle close failures by directly unwinding known tun_dst ownership, while SET_TO_MASKED actions require no external ownership management and simply truncate on close failure. This granular approach to resource management aligns with ATT&CK technique T1068, which involves privilege escalation through local system exploitation, ensuring that the vulnerability cannot be exploited for unauthorized access or privilege elevation within the kernel space.
The implementation maintains backward compatibility while strengthening security boundaries around nested attribute processing. By enforcing proper bounds checking on nested action attributes without restricting legitimate use cases, the fix preserves existing functionality while eliminating the dangerous condition that allowed malformed nested containers to cause structural inconsistencies in the flow action stream processing. This solution demonstrates effective balance between maintaining operational requirements and implementing robust security controls necessary for kernel-level network virtualization components.